The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device in which a light source element and an electronic component, which are mounted on a flexible wiring substrate, are attached to a frame in such a way that each of the light source element and the electronic component is placed in a housing concave portion provided in the frame of the backlight unit.
A thinned liquid crystal display device is used as a display device for various types of portable information devices such as mobile phones, personal digital assistants (PDA), digital cameras, and multimedia players.
A conventional liquid crystal display device will be described with reference to FIGS. 1A to 4.
First, an example of the conventional liquid crystal display device will be described with reference to FIGS. 1A and 1B.
Note that FIG. 1A shows a cross section of the end portion in which a light source element of a liquid crystal display device is provided, and FIG. 1B shows the liquid crystal display device seen from the left in FIG. 1A.
The liquid crystal display device includes a liquid crystal display panel 1, and a backlight unit provided on the back surface of the liquid crystal display panel 1.
The liquid crystal display panel 1 is formed by a pair of transparent substrates 2 and 3, with the space between the transparent substrates 2 and 3 filled with liquid crystal. In the transparent substrate 2, transparent electrodes and thin film transistors are arranged to drive the liquid crystal. Then, color filters are arranged in the transparent substrate 3. Further, a lower polarizing plate 4 is provided in the transparent substrate 2, and an upper polarizing plate 5 is provided in the transparent substrate 3.
Further, an edge portion of the transparent substrate 2 extends beyond the transparent substrate 3. A driver 6 for driving the thin film transistors is mounted on the extending part.
The backlight unit includes a light emitting diode 7 as a light source element, a light guide plate 8 for guiding the light emitted from the light emitting diode 7 to the liquid crystal display panel 1 to be irradiated, an optical sheet group 9 for uniformly diffusing the light from the liquid guide plate 8 to the liquid crystal display panel 1, a reflective sheet 10 provided on the back surface of the light guide plate 8, and a frame-like frame 11 formed by a material having relatively high rigidity to accommodate the light source and the light guide plate.
One end portion of the flexible wiring substrate 12 is fixed to the transparent substrate 2 of the liquid crystal display panel. The flexible wiring substrate 12 is formed by a resin material and the like, and has flexibility. A circuit connected to the driver 6 is formed in the flexible wiring substrate 12. Further, the light emitting diode 7 and an electronic component 13 such as resistance and capacitor are mounted on the flexible wiring substrate 12.
The light emitting diode 7 and the electronic component 13 are provided on the back surface side of the flexible wiring substrate 12 in such a way that the light emitting diode 7 is located at the end portion, followed by the electronic component 13.
A first housing concave portion 14 that is open to an end surface of the liquid guide plate 8, as well as a second housing concave portion 15 are formed on the back side of the frame 11. At this time, the second housing concave portion 15 is located after the first housing concave portion 14. The light emitting diode 7 is placed in the first housing concave portion 14 so that the light emitting surface of the light emitting diode 7 faces the end surface of the light guide plate 8. Then, the electronic component 13 is placed in the second housing concave portion 15.
In other words, in the assembly operation of the liquid crystal display device, the flexible wiring substrate 12 is fixed to the liquid crystal display panel 1 in one end portion thereof and has a flat plate shape. The light emitting diode 7 and the electronic component 13 are mounted on this flexible wiring substrate 12. Then, the light emitting diode 7 and the electronic component 13 are placed in the housing concave portions 14 and 15, respectively, by bending the flexible wiring substrate 12 over one side portion of the frame 11. Then, the light emitting diode 7 is fixed to the inner surface of the housing concave portion 14 with an adhesive layer 16 such as double-sided tape.
Thus, the liquid crystal display device is assembled with the flexible wiring substrate 12 bent as shown in FIGS. 1A and 1B.
The configuration in which the light emitting diode 7 and the electronic component 13 that are mounted on the flexible wiring substrate 12 are placed in the housing concave portions 14 and 15, respectively, by bending the flexible wiring substrate 12 as described above, is a so-called full flat back in which the back surface of the liquid crystal display device is flat. Thus, there is an advantage that the configuration of the liquid crystal display device to be mounted on an actual device, such as a mobile phone, can be simplified.
However, since the flexible wiring substrate 12 is bent over the side of the frame 11, a repulsive force acts on the flexible wiring substrate 12 by the internal stress in the direction in which the end portion thereof is removed and floated from the frame 11 as shown in FIG. 2A by the arrow.
Thus, there is a problem that the mounting position of the flexible wiring substrate 12 is changed to displace the optical axis of the light emitting diode 7 to emit light to the light guide plate 8, so that the brightness of the liquid crystal display device is not as good as designed.
With the reduction in size and weight of portable information devices to be implemented, the liquid crystal display device is required to be reduced in thickness and the bending diameter of the flexible wiring substrate 12 tends to be reduced. Thus, the repulsive force on the flexible wiring substrate 12 increases, so that such a failure tends to be more obvious.
As a method for preventing this failure, as shown in FIGS. 3A and 3B, a cutout 17 is provided around the bent portion of the flexible wiring substrate 12 in order to reduce the repulsive force generated from the bent portion of the flexible wiring substrate 12.
However, there is a limit to the area of the cutout 17 in the flexible wiring substrate 12 due to the need to ensure the area for wiring, which has made it difficult to sufficiently reduce the repulsive force. Thus, as shown in FIG. 4, a jig 18 is used in the assembly operation of the liquid crystal display device. In this method, the bent portion of the flexible wiring substrate 12 is held by the jig 18 to form a permanent bend in the flexible wiring substrate 12. However, this method may cause the line formed in the flexible wiring substrate 12 to be broken by the jig 18.
The problem is that the thickness of the liquid crystal module increases in the configuration in which the end portion of the flexible wiring substrate with the light emitting diode overlaps the reflective sheet that is provided on the back surface of the light guide plate. In order to solve this problem, Japanese Unexamined Patent Application Publication No. 2009-216753 describes an invention to reduce the length of the end portion of the flexible wiring substrate FPC to face an end surface of the reflective sheet.
Japanese Unexamined Patent Application Publication No. 2008-203445 describes an invention to form a through-hole in the bent portion of the flexible wiring substrate to reduce the repulsive force caused by the bending of the flexible wiring substrate. However, this invention has the same problem as that in the method for forming the cutout 17 as shown in FIGS. 3A and 3B.